The objective of this research is to establish a genetic basis of male recombination-mutator system (Mr system) of Drosophila melanogaster, and to obtain better understanding of the roles of mutation and crossing over in evolution. Specifically, the following points will be studied more intensively. 1. Variegation in Mr activity. A phenomenon like position effect variegation was suggested for Mr activity which located near, or in the centric heterochromatin. This will be further studied with the aids of variegation suppressing systems. 2. Suppressor-X chromosome. Natural and artificial (cage) populations will be analyzed for the presence of suppressor-X chromosomes, and the position(s) of suppressor(s) in the X chromosome will be determined. Nature of suppressions will be analyzed. 3. Relationship between male recombination (Mr) and Segregation Distorter (SD) systems will be studied. Many similarities exist between these two systems. One interesting point is that, in natural populations, Mr and SD seem to be "exclusive" in their relative frequencies, i.e., when Mr frequency is high, SD frequency is low in that population (perhaps SD is absent), and when SD is present, Mr frequency becomes low. It may be that some of the suppressors of SD (or an element of SD) may work to increase the transmission frequency of Mr from heterozygous male parent, even higher than the Mendelian expectation of 0.5, thus the Mr tends to increase its frequency in a population. This point will be studied in detail.